The present invention relates to a method and apparatus for use in the pulp industry for improving the safety of a spent liquor recovery boiler, such as a soda recovery boiler, whereby the boiler comprises, for recovering chemicals and energy, a furnace cooled by water tubes and connected to the water/steam circulation system of the boiler.
Recovery and regeneration of cooking chemicals effected in the soda recovery boiler are an essential part of sulphate and other Na-based pulp manufacturing processes. Organic substances dissolved in the spent liquor are combusted thereby generating heat which is utilized on one hand, when converting the inorganic compounds contained in the spent liquor back to chemicals to be used in cooking and on the other hand when generating steam. The inorganic matter, i.e., ash, contained in the spent liquor, melts at the high temperature prevailing in the soda recovery boiler and flows as melt downwardly onto the bottom of the furnace and further out of the boiler into a dissolving tank. The soda recovery boiler also serves as a steam boiler, where heat released during combustion is recovered as steam, primarily by water tubes lining the boiler walls, and as high-pressure superheated steam of, e.g. 450.degree. to 480.degree. C./65 to 85 bar, by superheaters disposed in the upper section of the boiler. The temperature rises very high, often to over 1000.degree. inside the furnace of the soda recovery boiler, whereby the conditions in the furnace are highly corroding due to the temperature and the alkali and sulphur compounds contained in the spent liquor.
These special circumstances set constructive demands on the structure of the soda recovery boiler, such demands being different from those set on conventional power plant boilers. In the furnace structures presently used, the whole bottom and walls of the furnace are water-cooled in order to prevent the temperature at the tubes from rising to a level favourable for corrosion.
The bottom and wall tubes nowadays used in soda recovery boilers are usually of gas-tight welded membrane structure, where the tubes are welded together through fins. The fin width is usually 10 to 25 mm. The outer diameter of the tubes is generally 57 to 70 mm. In boilers of another type, the tubes are welded together side by side as a tangential structure with no large spaces therebetween. The furnace has been made gas-tight by welding the tubes together.
The boiler water, which is often highly pressurized and has a temperature of over 300.degree. C., is usually led below the bottom of the furnace, wherefrom it is distributed to the tubes of the bottom and the wall. In the bottom tubes, the water first flows towards the walls and further upwardly therefrom. In the wall tubes, the water normally flows only upwardly.
The water circulation system of the soda recovery boiler functions by-natural circulation. Proper water circulation is of a crucial importance to the operational safety of the boiler. If the water circulation is disturbed, it may result in overheating of the tube material, and consequently corrosion and tube damage. Great changes in the tube temperature as well as in the deposits of chemicals and ashes covering the tubes may cause disturbances in the water circulation. Especially in the bottom tubes, where water flows horizontally or nearly horizontally, a disturbance in the water circulation may be fatal.
A bed is formed on the bottom of the furnace, composed of material remaining in the spent liquor after the evaporable and easily combustible substances have been discharged during the pyrolysis stage. The bed contains, e.g., coal/coke, sodium and sulphur. The amount of the bed mass varies depending on process conditions. On the bottom of the bed, near the bottom tubes of the furnace, there is a layer of melt formed of inorganic chemicals. This layer of melt flows out of the furnace through an opening or openings in the furnace wall.
The lowermost portion of the layer of chemicals, which is in direct contact with the water-cooled bottom tubes of the furnace, is normally in a solid state due to the cooling effect of the tubes. This solid layer protects the tubes.
Formation of the solid layer onto the bottom of the furnace may be disturbed by temperature variation in the furnace, shallowness of the bed, unfavourable inclination of the bottom of the furnace or by poor cooling effect of the boiler tubes. There may be also other reasons why a solid layer of chemicals is not formed, which causes the danger of the bottom tubes of the furnace becoming overheated.
In some cases, the state of the layer nearest to the tubes varies between solid and molten states, Depending on the structure of the lower section and bottom of the boiler furnace or on combustion conditions, the conditions on the tubes may vary, whereby the cubes are sometimes covered by a solid and sometimes by a molten layer of chemicals, and sometimes at least a portion of them is totally without any protecting layer thereon.
For reasons described above, the bottom tubes of the boiler may be damaged. For example, fractures or cracks may appear, wherethrough water then finds it way and leaks out of the tubes into the chemical melt. This brings about a danger of a violent explosion between water and melt, i.e., a melt explosion. Such explosions sometimes occur and material damages may rise to millions of Finnmarks. Repairs of the damages may take months, which results in considerable production losses. Sometimes melt explosions have also claimed human lives.
A leak in furnace tubes may be caused by overheating as mentioned above, or by corrosion by chemicals, internal stresses in the tube material, fatigue of material or a combination of these, or for some other reason.
Because of the danger of explosion, the bottom tubes of the boiler have to be inspected frequently in order to discover in good time whether the bottom tubes have corroded or otherwise damaged and whether they therefore have to be partly or completely replaced or repaired.
Efforts have been made to decrease the danger of damage and explosion by using tube material which better resists corrosion, or by covering the bottom tubes by refractory material or coating them by corrosion resistant metal to be sprayed on the tubes. The bottom tubes used in soda recovery boilers are generally, for example, carbon steel tubes or compound tubes. The compound tube is made of carbon steel and coated with a thin, protective layer of austenitic steel. However, the danger of explosion or other damage has not been completely avoided. Inspections of boilers have revealed cracks and signs of corrosion also under the coating material. Unfortunately, the inspection itself may contribute to a damage to the tubes because the protective layer coating the tubes has to be removed for the time of inspection.
An object of the present invention is to provide a better method and apparatus than those described above for improving the safety of a recovery boiler.
Primarily, it is an object of the present invention to provide a structure forming the lower section of a recovery boiler furnace, which structure is not inclined to cause a melt/water explosion in case of damage.
It is a still further object of the present invention to provide a structure forming the lower section of the recovery boiler furnace, which is easier and faster to replace.
For achieving the objects mentioned hereinabove, it is a characteristic feature of the method of the present invention that cooling of the furnace bottom is provided by a separate cooling circulation system.
Correspondingly, it is a characteristic feature of the apparatus according to the invention that said apparatus comprises a furnace bottom formed of cooling tubes or cooling surfaces connected to a separate cooling circulation system.
The separate cooling of the lower section of the furnace may be limited to, e.g., the bottom level of the furnace but it may also partly extend to the wall section or wall sections of the furnace.
When cooling of the bottom of the soda recovery boiler is arranged by using a cooling circulation system separate from the boiler water/steam circulation system, it is possible to use cooling mediums other than water, for example, air, other inert gas or some fluid material such as molten zinc.
A separately cooled lower section of the furnace according to the invention is provided, e.g., so that the furnace bottom or the furnace bottom and part of the furnace walls together are separated from the boiler water circulation system. This separate section employs a cooling medium of its own, which may be some gas or liquid, which does not cause an explosion should it come into contact with furnace chemicals.
According to a preferred embodiment of this invention, a separately cooled lower section of the furnace may be constructed of tubes similar or nearly similar to those used for conventional, presently used furnace bottoms. Circulation of the cooling medium in the separately cooled bottom may be arranged so that the medium is supplied into the cooling tubes from the tube end at one wall of the furnace, wherefrom it then flows via the tubes to the tube end at the opposite wall. Thereafter, the medium is discharged via circulation pipes, circulation pipe or channel. From a separate circulation piping, the cooling medium is led preferably by a pump or a blower via a separate cooler back to the starting end of the cooling tubes.
A separately cooled lower section of the furnace may also be constructed of tubes larger or smaller than the tubes in the furnace itself or alternatively of tubes or canals, the cross section of which is square or rectangular.
It is also possible to lead the cooling medium from more than one side to the cooling tube section of the bottom. Several alternative circulation or flow arrangements of the cooling medium are possible in the lower section of the furnace by arranging the tubes in a suitable manner.
The cooling medium may be led, for example, to the main distribution chamber and/or distribution chambers in the center of the bottom or to some other place between the wall and the center of the bottom, wherefrom the cooling medium is then distributed over the entire bottom section.
The temperature of the medium flowing in the separately cooled lower section of the furnace is maintained relatively constant by regulating the cooling effect of the cooler disposed in the circulation piping. The temperature of the cooling medium flowing in the separate cooling circulation system according to the invention is preferably controlled so that it causes the thermal expansion of the separately cooled lower section of the furnace to correlate with the thermal expansion of the pressurized furnace having water circulation, i.e, no sealing problems exist between the separately cooled lower section of the furnace and the other furnace structure and no gas or chemical leakages occur between the parts of the apparatus.
The arrangement of the invention provides a boiler bottom structure which does not cause an explosion if a tube damage occurs and cooling medium comes into contact with the chemical melt inside the furnace.
A further advantage of the arrangement for separate cooling is that the separate cooling tubes of the lower section of the furnace are partly or completely replaceable separately so that the actual boiler water circulation pipes need not be touched.
In accordance with the invention, a still further advantage is gained, which comprises that the cooler disposed in the circulation piping may be advantageously used for preheating of the combustion air to be supplied to the boiler. On the other hand, if air is used as a cooling medium, the air heated in the cooling circulation system may be directly used as combustion air in the furnace.